The overall objective of our work is the application of a non-equilibrium thermodynamic approach to determine physico-chemical characteristics of rabbit thoracic aorta with respect to porosity and selectivity, eventually leading to accumulation of low density lipoprotein and to atherosclerosis. We have already investigated these properties for flux across the full thickness of rabbit thoracic aorta in vitro. We now propose to extend these studies to determine the porosity and selectivity of the endothelium-intima layer for different solutes by measuring in a perfusion system the water uptake across the endothelium due to the osmotic gradients. The endothelial integrity, which may be difficult to maintain in vitro, particularly using previous techniques, will be monitored by the silver-nitrate stain microscopy technique and attempts will be made according to the current state of art to maintain endothelial integrity. Vasoactive agents like angiotensin and norepinephrine have been suggested to be involved in accelerating lipid accumulation by transiently forming endothelial gaps (the 'trap-door' effect). One of the main objectives of the proposed work will be to characterize, by dynamic permeability experiments, the effects of vasoactive agents on the porosity and selectivity of the aortic wall to molecules of different size. Agents like angiotensin II, norepinephrine, theophylline and Ca ions antagonists will be used at concentrations which are optimal in their effect on aortic smooth muscle cell relaxation and contraction.